The present invention relates to a transparent communication network for optical-signal transmission, having a ring-shaped structure, comprising a double communication line and nodes located along the line for adding and dropping signals related to one or more communication channels, from and into the ring respectively.
In the network, generally along optical fibres, signals corresponding to different channels, each having a different wavelength, travel together, according to the so-called wavelength division multiplexing (or WDM) technique. The components of one signal having a wavelength corresponding to one channel are dropped from the network and added thereinto in an optical form, at the nodes. The network enables transmission of optical signals without intermediate conversions to the electric form and it is therefore transparent to the particular structure of the elementary information to be transmitted (usually electric signals in a digital form).
In optical-signal transmitting networks the amount of the exchanged information is very high. For this reason, a failure can have very severe consequences in that a very high number of subscribers can be deprived of the information flow.
Typical failures may be breaking of an optical fibre, because said fibre can be, for example, accidentally cut by operators not aware of the presence of same, and a cutoff in the operation of the whole node, due for example to a fire, lack of electric energy or failure of one component.
In order to face such a situation in an automatic manner and within sufficiently short periods of time so that the information flow is not interrupted, self-healing-ring networks have been conceived.
In these networks the different nodes are connected with each other- by two optical-fibre lines, closed upon themselves to form a ring: a primary ring line (also referred to as external or working ring) and a secondary ring line (also referred to as internal or protection ring). Under normal conditions signals travel in one direction alone on the primary line and are dropped from and/or added into the different nodes depending on the wavelength.
In the case of a node failure, or breaking of a primaryline optical fibre between an upstream and a downstream node, continuity is restored by diverting the signal flow from the primary to the secondary line in the node which is upstream of the breaking (with respect to the signal travel direction in the primary line) and from the secondary to the primary line in the downstream node. In the secondary line, signals travel in the opposite direction with respect to the primary line. In order to perform these diversions in an automatic manner from one ring line to another, two so-called xe2x80x9c2xc3x972xe2x80x9d directional switches are provided for use in the nodes, which directional switches are optical four-way components having two inputs and two outputs. In a normal configuration, the first output is optically connected to the first input and the second output is optically connected to the second input, whereas in a switched-over configuration the first output is optically connected to the second input and the second output is optically connected to the first input.
A node of this type is described for example in an article by S. Merli, A. Mariconda and R. de Sanctis entitle xe2x80x9cAnalisi e dimensionamento di un anello ottico trasparente per sistemi D-WDM, con funzioni di riconfigurazione automatica in caso di rottura dell""anello e di Drop-Insert locale dei canalixe2x80x9d (Analysis and dimensioning of a transparent optical ring for D-WDM systems having functions of automatic reconfiguration in case of ring breaking and local Drop-Insert of the channels), Atti del Convegno FOTONICA ""95 (FOTONICA ""95 Meeting Records), Sorrento, IT, May 1995. It enables signals to be diverted to the secondary ring line in case of breaking of the primary ring line. In addition, it enables the optical user unit of the node (typically a wavelength-selective optical switch, for adding and dropping the signals of a channel having a predetermined wavelength) to be bypassed in case of failure of same, while saving transmission between the remaining network nodes.
Networks of this type require a central unit capable of recognizing and localizing an occurred failure (for example, through signaling of the lack of signal by the node located downstream of the failure, which can carry out switching of the optical signal to the service line in a self-governing manner) and of sending to the node upstream of the failure, the switching-over command so that it can receive signals from the service line. For this reason it is necessary for the central unit to communicate with each of the nodes, also and above all in case of failure in the communication line. Therefore emergency lines for connection between the nodes and the central unit need to be arranged, which lines must be independent of the ring line for communication between the nodes. These emergency lines (that can be optical, electrical, or radio link lines or lines of any other type) together with the central unit involve an important complication in the communication network.
In addition, in network architectures of the above stated type, the time necessary for transmitting the information about the occurred failure to the central unit, processing the information by said unit and transmitting the switching over command to the node upstream of the failure is to be added to the intervention time by the directional switches, establishing the overall time for recovery of the network functionality after a break. This overall recovery time can be much higher than the intervention time of a directional switch, presently reaching values on the order of some milliseconds in switches of the acousto-optical or magneto-optical type or some tens of milliseconds in switches of the mechanical type.
An optical self-healing-ring network is also described in an article xe2x80x9cA uni-directional self-healing ring using WDM techniquexe2x80x9d, by E. Almstrom et al., ECOC ""94 Conference Proceedings, Florence, IT, 25-29 Sep., 1994, vol. 2, pages 873-875. The optical network contemplates the use in the nodes of optical switches having more than one input and more than one output. The network is shaped so that the nodes can close the ring on the protection fibre, as soon as they detect a break along at least one of the fibres.
According to one aspect, the present invention relates to an optical self-healing-ring communication network comprising:
one optical communication line forming a first closed optical path;
at least two optical-signal add/drop nodes optically connected along said optical communication line;
a second optical communication line forming a second closed optical path and optically connected to said optical-signal add/drop nodes;
in which at least one of said nodes comprises controlled selection means for selectively dropping said optical signals from one of said first and second communication lines,
wherein at least one of said optical-signal add/drop nodes further comprises means for the simultaneous input of at least one optical signal into said first communication line and into said second communication line.
In particular, defined in the optical ring communication network is a first and a second travel direction of said optical signals relative to the position of said optical-signal add/drop nodes, said first and second directions being opposite to each other and said network comprises means for the simultaneous input of at least one optical signal in said first direction along said first communication line and in said second direction along said second communication line.
Preferably the optical ring communication network comprises one pair of optical-signal add/drop nodes, optically connected with each other, in which the signals transmitted between the nodes of said first pair have a first wavelength, and at least one of said first and second optical paths comprises a second pair of optical-signal add/drop nodes, optically connected with each other, the signals transmitted between the nodes of said second pair having a second wavelength different from said first wavelength.
In particular, in the optical self-healing-ring communication network according to the present invention at least one of said optical-signal nodes comprises:
one optical-signal add/drop unit, connected in series by respective line input and output ports to said first optical path, further having respective signal add and drop ports;
a second optical-signal add/drop unit, connected in series by respective line input and output ports to said second optical path, further having respective signal add and drop ports;
a controllable optical switch having a first and a second selectable inputs connected to the signal drop ports of said first and second optical-signal add/drop units respectively, and one output;
a terminal line unit having an optical input connected to said output of said optical switch and having two optical outputs connected to the signal add ports of said first and second optical-signal add e drop units;
means for detecting the presence of the optical-signal at said drop ports of said first and second optical-signal add/drop units, operatively connected to said controllable optical switch for selection of a respective one of said first and second inputs.
Preferably, said first and second optical-signal add/drop units each comprise a demultiplexing unit of said received optical signals at the respective wavelengths and an optical-signal multiplexing unit at the respective wavelengths, and the outputs of said demultiplexing units corresponding to the wavelengths included within said bypass band are selectively connected to corresponding inputs of said multiplexing units.
In a second aspect, the optical self-healing ring communication network according to the present invention comprises a firs t and a second optical self-healing-ring telecommunication network, as previously defined, wherein at least one optical node of said first network is optically connected to at least one optical node of said second network.
In another aspect, the present invention relates to an optical self-healing-ring telecommunication method in an optical telecommunication network, which comprises feeding at least one optical signal from a first add/drop node in a first closed optical path included within said network, to a second add/drop node serially connected in said first closed optical path, characterized in that it further comprises the steps of:
simultaneously feeding said optical signal from said first add/drop node to said first closed optical path and to a second closed optical path included in said network, said second optical path comprising said second add/drop node serially connected thereinto, and
selectively receiving said at least one optical signal in said second add/drop node from one of said first and second closed optical paths.
In particular, said optical telecommunication method is characterized in that said step of feeding an optical signal to said first and second closed optical paths comprises feeding said signal in two opposite directions with respect to said first add/drop node.
In particular, said step of selectively receiving said optical signal in said second add/drop node comprises:
detecting the presence of said signal in said first closed optical path in said second node, and
operatively switching over reception from said first closed optical path to said second closed optical path in the absence of signal in said first closed optical path.
In a preferred embodiment thereof, the optical telecommunication method according to the present invention is characterized in that said first and second optical-signal add/drop nodes form a first pair of nodes optically connected with each other, wherein the signals transmitted between the nodes of said first pair have a first wavelength, and at least one of said first and second optical paths comprises a second pair of optical-signal add/drop nodes, optically connected with each other, the signals transmitted between the nodes of said second pair having a second wavelength different from said first wavelength.
In particular the optical telecommunication method comprises adding and dropping optical signals having a wavelength included within a communication band in a first and a second add/drop units, wherein said first and second units comprise optical-signal drop outputs and optical-signal add inputs and said step of adding and dropping optical signals comprises:
receiving optical signals from said first and second closed optical paths, respectively;
sending to said drop outputs, the received optical signals of a wavelength extending within a predetermined dropping band included in the communication band;
sending to said first and second closed optical paths respectively, the received optical signals of a wavelength extending within a predetermined bypass band, included within the communication band and having no overlap with said dropping band;
sending to said first and second closed optical paths respectively, the optical signals present at said add inputs, of a wavelength included within said dropping band.